The present invention relates to semiconductor laser devices, and more particularly to vertical cavity surface emitting lasers (VCSELs).
VCSELs are important semiconductor laser sources for many applications, including telecommunications. Though VCSELs formed on a chip in arrays are individually coherent, the light from each individual VCSEL is not coherent with respect to the others because their phase and wavelength differ slightly, and are therefore uncorrelated. For such an incoherent array consisting of N elements producing some power P, the on-axis power in the far field is xcx9cNP. However, if the array as a whole can be made to operate coherently (i.e., each individual VCSEL is coherent with the others), and in phase, the on-axis power in the far field is N2P and the width of the radiation pattern is reduced by xcx9c1/N (compared to an incoherent array). This high on-axis far field power is required in laser applications such as free-space optical communications and laser radar where a large amount of power is required at a distance, or in applications such as laser welding, laser machining, and optical fiber coupling that require high power focused to a small spot.
The monolithic power combining approach described in the present application brings the proven technological and economic benefits demonstrated by combining transistors, capacitors, and resistors into large scale integrated circuits to integrated photonic circuits consisting of lasers, optical waveguides, and grating couplers. Presently, the cost of high power (500 to 1000 W) Nd:YAG and CO2 lasers exceeds $100/Watt. Coherent VCSEL arrays can reduce this cost by one or two orders of magnitude. The cost advantage of VCSELs has been realized in local area network (LAN) markets where low power ( less than 0.005 W) VCSEL based transceivers selling for about $100 dominate the market, having displaced more expensive edge emitting devices.
Other applications, such as optical pumps at 980 nm for erbium doped amplifying telecomm fibers will benefit from low cost lasers with increased power (0.05 W to 1 W). Such powers cannot be achieved with individual VCSELs but could easily be achieved with small VCSEL arrays. Aside from these commercial and economic applications, coherent arrays of VCSELs have far-reaching significance because they have the potential to deliver very high power ( greater than  greater than 1 W) over a wide variety of wavelengths. Such a tool is certain to accelerate progress in medicine, communications, manufacturing, and national defense.
High-Power Coherent Array of VCSELs
The present application discloses an array of VCSELs which demonstrate coherent operation with respect to one another. In a preferred embodiment, coherent operation is obtained by using a waveguide which optically couples the lasers of the array. The waveguide couples the lasers with a periodic grating structure formed to capture a small part of the light from an individual laser, removing it from that laser""s cavity, while also adding light to that laser""s cavity from other lasers of the array. Thus the common waveguide both removes light from and adds light to each laser in the array, transferring enough optical power in the coupling exchange for phase locking to occur.
In one preferred embodiment, the common waveguide is located outside the monolithic VCSEL array, not being fabricated on the same wafer. In another preferred embodiment, the common waveguide is fabricated on the wafer with the VCSELs, and is located outside the cavity of the lasers, above them on the wafer structure. This approach avoids introducing complexity and losses into the individual VCSEL structure. In another preferred embodiment, the common waveguide is fabricated within the cavity, between the two reflectors of the VCSEL structure.
Advantages of the disclosed methods and structures, in various embodiments, can include one or more of the following:
1xe2x80x94provides a precise, controlled amount of coupling between array elements, independent of the element size and spacing;
2xe2x80x94can include phase adjustors to assist in beam formation and electronic beam steering (the phase adjustors can be a VCSEL element operating at below lasing threshold, where variations in current correspond to significant changes in the effective index of refraction in the gain region, and therefore the effective optical length of the corresponding waveguide segment.